Brillouin scattering in an optical fiber is changed depending on a strain applied to the optical fiber. There has conventionally been devised a technique for measuring distributed strain along the optical fiber by using this phenomenon. This measuring technique enables the level of strain to be measured by measurement of a frequency change in Brillouin scattered light, and also enables distorted part of the optical fiber to be identified by measurement of the time until the Brillouin scattered light returns. Accordingly, when optical fibers are wired in all directions on the constructions such as bridges and bridge piers, buildings, and dams, and/or on the materials such as wings and fuel tanks of an airplane, the distribution of strain applied to these constructions and/or materials can be revealed. Based on such distribution of strain, deterioration and/or secular change in materials and structures are revealed. Accordingly, this measurement technique is attracting attention as a technique useful for disaster and/or accident prevention (see, for example, Patent Literatures 1 and 2).
A description is now given of the principle of the Brillouin scattering. When light is incident on a general optical fiber, glass molecules in the material of the optical fiber thermally oscillate and generate ultrasonic waves, which include an ultrasonic wave having a wavelength half the wavelength of the incident light. Periodic change in a refractive index of the glass caused by the ultrasonic wave function as a Bragg diffraction grating for the incident light, and reflects the light backward. This is how the Brillouin scattering phenomenon works. While the reflected light is Doppler-shifted depending on the velocity of the ultrasonic wave, the amount of frequency shift varies depending on expansion and contraction strain applied to the optical fiber. Accordingly, the strain can be detected by measuring the shift amount.
As a typical technique to measure the distribution of such Brillouin scattering along a length direction of the optical fiber, a Brillouin optical correlation domain analysis (BOCDA) method is known as disclosed in Patent Literature 1 and the like.
However, the Brillouin scattering in the optical fiber depends not only on strain but also on temperature. At measurement sites where temperature changes, precise measurement is not available. Accordingly, in order to solve such a problem, an optical fiber property measuring device has been proposed which applies the above-stated BOCDA method to an optical fiber under test having a polarization retention property and which also measures a spectrum of reflected light generated by the Brillouin dynamic grating which is a phenomenon relevant to the Brillouin scattering at the same time, so that both variation of the temperature and the strain can precisely be measured based on the result of these two measurements (see, for example, Non Patent Literature 1).